The present invention relates, in general, to a backflow prevention device in combination with a plasticizing and injection screw axially movable in a plasticizing cylinder of an injection molding machine.
Injection molding of thermoplastic material involves the use of a screw which is received in a plasticizing cylinder for movement in an axial direction and rotates therein. Friction caused by the movement and rotation of the screw and exposure to heat generated by heating bands mounted on the outside of the plasticizing cylinder result in a melting of the plastic granulate. The screw operates also as a plunger to inject the plastic melt into a cavity of a molding tool, as the screw is moved in axial direction in the plasticizing cylinder. A backflow of plastic melt during the injection process, as the non-rotating screw moves forward, is prevented by the use of a backflow prevention device or non-return valve. It will be understood by persons skilled in the art that the terms “backflow prevention device” and “non-return device” are used synonymous in the disclosure.
International patent publication WO 00/23248 describes the application of a non-return valve which includes a locking sleeve or a locking ring positioned at the leading end of the screw. During the injection process, the locking ring is pressed against a pressure ring surface of a base body arranged at the head portion of the screw. The locking ring is lifted off the pressure and stop surface and pressed against the stop surfaces of blades arranged at the head of the base body, during metering, i.e. as the screw reverses and rotates. The locking ring is freely movable relative to the screw and is prevented from rotating during metering as a consequence of friction upon the wall of the plasticizing cylinder or rotates at little speed. As a result, friction is encountered between the locking ring and the stop surfaces of the blades.
German Pat. No. DE 32 47 272 C2 describes a ring-type backflow prevention device having balls disposed between a locking sleeve and the stop surface of a base body arranged at the screw head. These balls, which are distributed about the circumference, serve as spacers between the locking sleeve and the base body and at the same time operate as ball bearing to thereby provide better friction conditions between the base body of the backflow prevention device and the locking sleeve.
Conventional ring-type backflow prevention devices suffer shortcomings because the closing process is normally subjected to stochastic influences so that scattering is caused during the injection process. Moreover, ring-type backflow prevention devices exhibit comparably poor response or closing behavior during the injection process.
A different type of backflow prevention devices are so-called ball-type backflow prevention devices which include one or more balls received in bores in a cylindrical portion of the screw head and operate like ball check valves. Examples of such types of backflow prevention devices include German patent publication DE 198 19 808 A1 or German Pat. No. DE 1 266 486. The bores have each two regions of different diameter and a transition zone which serves as sealing surface. The diameter of the bore region positioned upstream of the transition zone, as viewed in transport direction of the melt, is greater than the diameter of the downstream region to provide a support for the ball to serve as locking member. During the metering process, plastic melt flowing through the narrow bore lifts the ball away from the sealing surface to clear the passage for melt to flow to the region of greater diameter. Melt can thus flow through the bore past the ball or balls into the anterior screw space. To prevent the balls from leaving the wider region of the bores, the bores extend slantingly in the direction toward the inner wall of the plasticizing cylinder. Other approaches propose a construction of bores to extend into the anterior screw space, whereby stays or like structures are provided in the bores to prevent an escape of the balls from the bore.
Although ball-type backflow prevention devices exhibit better response and closing behavior than ring-type backflow prevention devices, they suffer shortcomings because measures must be taken to minimize wear of the cylindrical portion of the screw head upon the corresponding inner wall of the plasticizing cylinder through selection of a sufficiently sized gap width between these components. This is especially relevant, when the screw has a great diameter. As a result of this measure, a leakage flow is encountered from the anterior screw space through the gap to the rear, after closing of the ball, so that a required holding pressure can no longer be applied during the injection molding operation. Dimensioning of the gap width further requires consideration of the shearing sensitivity of the plastic materials. When the gap is too narrow, melt film located in the gap would be exposed to great shearing forces, resulting in decomposition, color changes or changes of additives, such as, e.g., oxidation of flame-inhibiting additives. Thus, modified melt components reach the cavity together with clean melt, so that the finished article has poor quality. On the other hand, when the gap is too great, the afore-mentioned leakage flow is encountered. As a consequence, the configuration of a ball-type backflow prevention device requires a compromise between the need for a small leakage flow (smallest possible gap width), on one hand, and small wear of the screw and small shearing forces in the plastic material (greatest possible gap width), on the other hand.
It would therefore be desirable and advantageous to provide an improved backflow prevention device which obviates prior art shortcomings and which exhibits a good response and closing behavior, while preventing leakage flow from the anterior screw space as well as undesired shearing forces upon the plastic material.